The phrase "virtually mandatory" is correct and can be used in written English.
It means something that is almost always required or necessary. Example: It is virtually mandatory for all employees to attend the safety training session.
Attendance at these camps is now virtually mandatory.
Such courts and trials follow a practice of virtually mandatory pretrial detention for extended periods.
He was charged with drug offenses that would make his deportation virtually mandatory.
Today, Edwards talked about the need for balance and made it virtually mandatory that the ground game start churning.
"It used to be an optional thing, and now, especially among girls, it's virtually mandatory," she said.
The new Barnes will include many of the features that have become virtually mandatory in the museum world today — conservation and education departments, temporary exhibition space, auditorium, bookstore, cafe — making it four times the size of the old Barnes.
AGES 3-6 -- Parental death is virtually a mandatory ingredient in children's films, or so it seems, but here the deaths are relatively low profile.
But an acceleration of the process could also add an element of uncertainty about the safety of a vaccine that might be mandatory for virtually all schoolchildren.
Nutrition labeling is mandatory for virtually every packaged food, but not for meat and poultry, which are regulated by the Agriculture Department.
To reduce building emissions, the new proposal makes energy conservation measures mandatory for virtually all buildings through a process called "recommissioning".
